1. Field of the Invention
The present invention relates to a microwave oven and, more specifically, to a microwave oven in which rush current to a high voltage transformer is suppressed.
2. Description of the Background Art
A microwave oven heats food, by oscillation of high frequency radio wave from a magnetron. In a microwave oven, a high voltage transformer is mounted to supply power to the magnetron.
As a magnetic material is used for the core of the transformer, the transformer has hysteresis characteristics in magnetic field H and magnetic flux density B (see R1 of FIG. 11A). FIG. 11B shows time change of voltage waveform (R2) and magnetic flux waveform (R3: current waveform) of the high voltage transformer.
In the transformer, the phase of the magnetic flux is advanced by xcfx80/2 from the voltage phase. As can be understood from FIG. 11A, excitation magnetic flux generated in the high voltage transformer changes from xe2x80x94"PHgr" to +"PHgr". As can be understood from FIG. 11B, when power supply to the high voltage transformer starts at a timing of point P or point Q where magnetic flux attains zero, that is, at the phase providing maximum voltage, the rush current generated in the high voltage transformer can be suppressed low.
Actually, power supply to the high voltage transformer is controlled by controlling ON/OFF of a power relay. It is often the case, however, that because of variations in the power relay operation time, the timing of power supply to the high voltage transformer varies, resulting in a large rush current generated in the high voltage transformer.
Japanese Patent Laying-Open No. 2-160393 discloses a technique to shorten the operation time of the power relay, in order to suppress variation of the operation time of the power relay.
The technique described in this laid-operation application, however, cannot by itself fully suppress the rush current at the start of power supply to the high voltage transformer.
Through intensive study, the inventors of the present invention came to a conclusion that in order to suppress rush current at the start of power supply to a high voltage transformer, it is necessary to control more accurately than in the prior art, the timing of power supply from an AC power source to the high voltage transformer. Further, it was found that the rush current at the start of power supply to the high voltage transformer is influenced by the state of residual magnetic flux at the high voltage transformer left when power supply to the high voltage transformer is stopped immediately before.
The present invention was made in view of the foregoing, and an object is to provide a microwave oven in which rush current in the high voltage transformer can be suppressed.
According to an aspect, the present invention provides a microwave oven including a magnetron heating food, a high voltage transformer supplying a high voltage to the magnetron, and a control unit enabling switching between presence/absence of power supply from a prescribed AC power source to the high voltage transformer and capable of detecting a power supply voltage phase, that is, the phase of the voltage supplied by the prescribed AC power source. The control unit starts power supply to the high voltage transformer at the power supply voltage phase that corresponds to the phase of the power supply voltage at the time when power supply to the high voltage transformer is stopped last time.
According to the present invention, from the phase of the power supply voltage at the time when power supply to the high voltage transformer is stopped last time, the state of residual magnetic flux at the start of power supply in the high voltage transformer can be expected.
Therefore, power supply to the high voltage transformer is possible while considering the state of residual magnetic flux, and hence, rush current in the high voltage transformer can be suppressed.
According to another aspect, the microwave oven in accordance with the present invention includes a magnetron heating food, a high voltage transformer supplying a high voltage to the magnetron, and a control unit enabling switching between presence/absence of power supply from a prescribed AC power source to the high voltage transformer and capable of detecting a power supply voltage phase that is the phase of the voltage supplied by the prescribed AC power source. The control unit stops power supply to the high voltage transformer at a time point when the power supply voltage phase attains to a prescribed phase, if a condition to terminate heating by the magnetron is satisfied.
Thus, power supply to the high voltage transformer can be stopped at such a phase of the power supply voltage that suppresses rush current at the start of next power supply to the high voltage transformer.
Preferably, in the microwave oven in accordance with the present invention, the control unit starts power supply to the high voltage transformer at a time point when the power supply voltage phase is at a specific phase, if the power supply voltage phase at the time point when power supply to the high voltage transformer is stopped last time could not be detected. Accordingly, regardless of the state of residual magnetic flux in the high voltage transformer, power supply to the high voltage transformer can be started at such a power supply voltage phase that can to some extent suppress the rush current.
According to a still another aspect, the present invention provides a microwave oven including a magnetron heating food, a high voltage transformer supplying a high voltage to the magnetron, a relay capable of assuming a first state in which a prescribed AC power source is electrically connected to the high voltage transformer and a second state in which connection between the prescribed AC power source and the high voltage transformer is canceled, and a control unit controlling the operation of the relay. The control unit causes the relay to operate preliminarily from the second state toward the first state to almost establish the first state, before switching the relay from the second state to the first state.
According to the present invention, as the preliminary operation is performed, even when the relay has not being set to the first state for a long period of time, the state of the relay can be changed smooth when the control unit operates the relay from the second state to the first state. In other words, the state of the relay can always be changed smooth.
Therefore, the time period from issuance of a command from the control unit to set the relay to the first state until the relay actually assumes the first state can be made constant. Therefore, the timing of power supply to the high voltage transformer can be precisely met.
In the microwave oven in accordance with the present invention, it is preferred that the control unit causes the relay to execute the preliminary operations a number of times.
This enables smooth operation of the relay with higher reliability.
In the microwave oven in accordance with the present invention, it is preferred that the control unit returns the relay to the second state, after the relay performed the preliminary operation.
This surely avoids such a situation in that power is accidentally supplied from the prescribed AC power source to the high voltage transformer, when only a preliminary operation of the relay was intended.
Preferably, the microwave oven in accordance with the present invention further includes a heating chamber accommodating food, a display unit capable of displaying the state of the microwave oven, and a temperature detecting unit detecting the temperature in the heating chamber. The manner of display on the display unit is preferably changed in accordance with the temperature detected by the temperature detecting unit, after the end of heating operation by the magnetron.
Thus, even after the end of heating operation of the microwave oven, it is possible to provide a warning to the user.
Preferably, in the microwave oven in accordance with the present invention, the display unit is capable of displaying symbols, and the color of the symbols at the display unit changes in accordance with the temperature detected by the temperature detecting unit, after the end of the heating operation by the magnetron.
This allows the user to readily recognize the temperature of the heating chamber.
Preferably, in the microwave oven in accordance with the present invention, the display unit is implemented by a liquid crystal display device with a back light, and display color of the back light changes in accordance with the temperature detected by the temperature detecting unit, after the end of the heating operation by the magnetron.
This allows the user to readily recognize the temperature of the heating chamber.
Preferably, in the microwave oven in accordance with the present invention, the display unit ends the display operation, provided that the temperature detected by the temperature detecting unit attains to a prescribed temperature or lower, after the end of the heating operation by the magnetron.
Thus, at a time point when warning to the user becomes unnecessary, the display operation of the display unit ends. In other words, in the microwave oven, power consumption can be suppressed, while necessary warning is given to the user.
Preferably, in the microwave oven in accordance with the present invention, the control unit is fed by a prescribed power source, and preferably, power supply to the control unit is stopped provided that the temperature detected by the temperature detecting unit attains to a prescribed temperature or lower.
Accordingly, at the time point when warning to the user become unnecessary, power supply to the control unit ends. In other words, in the microwave oven, power consumption can be suppressed while necessary warning is given to the user.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.